For example, I have 80/90 slots for squads in my city. On the map I have an army with 50 slots of soldiers.
Could I enter with all of this army to my city and rush to an enemy, when somebody attacks my castle ?

Commander - unfortunately not, only the 90 units you actually have on your homeland will participate in the battle. Later in the game it`s best to leave more military units in your town, and transfer your peasants and livestock in the army with Army camp set-up (they also generated resources). Also keep in mind that if you have like 6+ various military buildings, you should be able to train 20-30 units during the time it takes the enemy to siege your town.

Thanks! It's really interesting information. I will use it . Sure, my city will not look so good without all these pigs running through the streets, but some archer squads instead, could save my city from an enemy.